Steam sterilization is a well known technique for destroying various microorganisms. Typically it is carried out at temperatures of from about 120.degree. C. to 140.degree. C. and steam pressures of from about 1 to 2 kilograms per square cm in an autoclave. The time required for such sterilization varies from about 2 minutes at 140.degree. C. to about 15 minutes at 120.degree. C.
It is highly desirable to have some method or device that indicates that a sterile condition has been achieved in the autoclave. The method or device is preferably accurate, fast, and inexpensive, and it must be dependable. Although several techniques have been previously utilized to indicate the attainment of a sterile condition, none have proven entirely satisfactory.
For example, spore strips have been placed in the autoclave with the material to be sterilized. These strips contain microorganisms that are difficult to destroy. After completion of the sterilization cycle the strips are examined to determine if the spores can reproduce in a suitable culture medium. If they cannot, sterilization is deemed adequate. While this technique is accurate, it is lengthy (i.e., it requires from several hours to several days to determine if the spores can reproduce); there is variation between lots of spores; the spores become less resistant to heat upon storage; the test is expensive to perform.
Attempts have also been made to indicate the attainment of sterility by the use of chemicals that either melt at a given temperature or that undergo a chemical reaction that brings about a color change. However, these techniques only indicate whether or not the autoclave has been held at a temperature for a time sufficient to cause melting or the chemical reaction to take place. Additionally, they generally do not give an absolute yes/no indication. Thus, the signal given by each must be interpreted so as to determine the degree of melting or color change.
Yet another attempt to provide a satisfactory indicator utilizes an organic compound that has a melting point higher than the sterilization temperature to be monitored, a wick that absorbs the liquid form of the organic compound by capillary action, and a steam-permeable cover that totally encloses the organic compound and the wick. Such a device is disclosed in U.S. Pat. No. 3,981,683.
In operation, the organic compound begins to melt when exposed to a predetermined temperature in the presence of steam. The wick draws the liquid organic compound away. A sterile condition is deemed achieved when the liquid has travelled up the wick a predetermined length.
This device depends upon the permeability of the cover to steam and the capillary action of the wick in order to be effective. Consequently, its reliability is dependant upon the lot-to-lot variability of the materials used in both the cover and the wick. Furthermore, it requires that the user interpret the amount of wicking that has occurred in order to determine if sterility has been achieved.